1. Field of the Invention
The present invention relates to a femoral prosthesis to be applied in total hip arthroplasty (THA). More specifically, new femoral prosthesis is a necklocking femoral component. It is anchored on the outside of retention neck and the trochantic bed of the femur and mechanically fastened during the operation.
2. Description of the Prior Art
The human hip joint comprises a socket or an acetabulum and a ball or femoral head. The femoral head is attached to the femur through a neck with reduced diameter and is received within the acetabulum for universal pivotal movement. In natural femur, most of loading force of hip is located on the head and distributed through the hard cortex of the femoral neck. It would be further conducted to the femur cortical bone and the joint of knee. If the femoral head becomes diseased or damaged, it can be replaced with a prosthetic femoral component. About 40 years ago, Prof. Sir John Chamley started a revolution in total hip replacement, a stem hip prosthesis. THA has been a tremendously successful surgical invention for patients with disabling arthritis of the hip. Quality of life is predictably improved following THA.
The implantable hip prostheses used comprise, in basic design, one or two piece(s) including a replacement head and femoral neck attached to an elongated metal stem that is adapted for receipt in a cavity formed in the proximal region of the femur. In practice of THA, the natural femoral head and neck has been completely removed, in order to open a canal on the proximal femur. In that model, most of compression force applied on the prosthetic femoral head would be directly conducted into the intramedullar canal of the proximal femur. Most of them are concentrated on the certain local area of the canal. Various patents of THA have been issued around this idea during past 30-40 years. Most of them have been fused on an improvement on shape of the stem for accurately fitting into intramedullar canal of shaft and a method to form a stable, longlasting prosthesis.
In early prior art, the stem of prosthesis was cemented within the femoral medullar cavity by use of a plastic cement, such as PMMA. After decades of practice, there were several infirmities associated with this kind of prosthesis installation. Typically, the cement would be deteriorated with age. In the meanwhile, the constant cyclic loading applied to the stem of the prosthesis by the action of simple walking causes cement to fracture and break loose from the femoral cavity, thereby requiring another operation to re-fix the prosthesis. Loosening of the cemented implants from canal was a common problem in THA and was being recorded with increasing frequency, leading eventually to the description of a condition called xe2x80x9ccement diseasexe2x80x9d. This xe2x80x9cdiseasexe2x80x9d seemed to be found with any of the nearly over 100 different prostheses in use, regardless of the size of the head, the shape of stem, the cementing technique used, or their modularity.
Because of problems associated with cementing a prosthesis hip within the femur, a cementless type of implant procedure had been developed and accepted by the orthopaedic community as a replacement. The cementless hip prosthesis includes a porous coating surface or coating materials, called hydroxyapatite (HA), over the length of the stem, such that natural growing bone after surgery penetrates into the porous surface or HA of the stem, thereby firmly fixing, at least in theory, the femoral stem within the femoral cavity. In general, such procedure requires that the patient would be relatively immobile for a number of weeks following the surgery, in order to allow the bone growth occur and thereafter restrict his physical activity until a firm bound is created. The period of time may involve a full twelve months. Secondly, micro-motion of the implant cannot be avoided in the immediate and early stages after surgery, which, more or less, causes the position or orientation changes of implanted prosthesis. The outcomes from such procedure would be largely depended upon the condition of patient in term of their health condition during the surgery period and activities thereafter. Even that, in best case, this procedure can not completely avoid problems that happened in the cement procedure. In addition, it becomes very difficult to remove a firmly bonded prosthesis, if a failure of prosthesis occurs after surgery. Another downside associated with existing devices and procedures is that, in order to avoid the loosening prosthesis, in prior art, the device has to be as much as perfectly fit into the medullar cavity in term of its orientation, position and the deepness at the medullar canal. From point of view of doctor, they have to spend most time during the surgery to carefully remove bone, which mostly is healthy and functioning, and open a canal in the cancellous bone in term of the precise position, shape and size of canal. It is a time consuming and skilful part in the operation. In fact, for most patients, who needs total hip replacement, their natural neck is functioning and is still in a good shape.
In addition, because design of the stem type devices and its operating procedure of THA would partially alter the structure and functions of the natural femoral system, somehow such surgery would make a damage on proximal femur and affect the growth of bone as well as quality of intact bone. In general, this procedure could not be recommended for young patients (younger than 50 years old), who has a damage on its femoral head, because of concerns about both durability of the prosthesis and most of side-effects and complications appeared in the regular stem type prosthesis, which could lead a need for multiple revisions in their lifetime.
Several previous studies have discovered that major reasons for the vertical migration and subsidence of the femoral component in hip replacement, which causes a likelihood loosening of the device, accurately, are: 1) From point of view of material properties, the cancellous bone is very different from one of cortical bone and metal. For example, the elastic modulus of the cortical bone, cancellous bone and stem (CoCrMo) are 17.3xc3x97109, 324.6xc3x97106 and 196xc3x97109 (Pa), respectively. In principle, function of the cancellous bone is not for supporting any load at all. Once the cancellous bone contact with hard surface of metal stem and is being loaded with force beyond its physiological limits, the plastic deformation happened, which would accumulate over extended period of time and manifest itself as migration of the prosthesis. 2) There are a change and difference on model of the force loading and distribution on the proximal portion of the femur between the intact femur and implanted stem prosthesis. According to Wolff""s law, changes in stress distribution through a bone eventually cause definite alteration in its internal structure. For example, the strain applied on femoral head is transferred along the length of the stem and compression force from femoral head is only focused on a small and local area of the medullas channel, which leads a local plastic deformation of the cancellous bone. 3) The current surgery of THA has generally required removal of entire head and neck portion as well as some hard outer cortical bone, in order to open the intramedullar canal and install the prosthesis. Such surgery causes a lot of anatomical changes on natural system, in term of blood circulation and supply to this area, so that it also leads further physical changes in proximal femur with respect of the bone quality after surgery. Those problems have been recognised for quite some time by orthopaedic community. Obviously, the fact of that problems are associated with the current design of the hip prosthesis, such as aseptic loosening, fatigue fracture, postoperative infection and stress shielding, could directly or indirectly relate to the stem-style design of the prosthesis with respect of the weakness on both biomechanical and physiological outcomes from the existing THA.
There are also several patents of femoral prostheses issued, which have non-stem construction of femoral prosthesis and intended to retain the femoral neck. Most of them focus only on the replacement of the defective femoral head, which eliminates overcutting healthy bone from patient. For example, U.S. Pat. No. 5,133,769 (Wagner) teaches the cap for a femoral head, which can be imbedded without the use of cement. U.S. Pat. No. 4,976,740 (Kleiner) describes an anchored femoral dome that can be abutted with a sculpted femoral head through mechanical fastens. U.S. Pat. No. 5,725,593 (Caracciolo) describes a total anatomic hip prosthesis which applied a cap on the defective femoral head with anchorage means fitting by pressure the acetabulum in the iliac fossa. Those technologies all have its limitation and might not be widely applied. Because the size of femoral head varies a lot from patient to patient, it is difficult to have devices for matching each individual one. In most case, the femoral neck, more or less, has been damaged, so that it can not fully support the load from new head at all, even though the head has been repaired.
In point view of practice, beside diseased femoral head, most of the surface and the subsurface bone tissue around proximal femur is actually healthy, particularly neck portion of femur for many case. In these situations, it is undesirable to remove the healthy portion of the femoral neck. It has been recognised in previous studies that the femoral neck, particularly the medial neck cortex of femur, plays a very important role in both loading and conducting compression force on proximal femur. Thus, the retention of the femoral neck and conducting the load through cortical bone of femur would be a very important strategy for designing a new prosthesis hip and developing a new procedure of its implantation.
In light of foregoing problems with prior art, particularly, of the stem-type femoral components and procedures, it is an object of the present invention to provide a new anatomic, neck locking femoral component for THA, by which patient do not need the femoral neck removed anymore. A hollow neck-shell, which supports the prosthetic femoral head, would be anchored onto the outside of retention neck and the trochantic bed of the proximal femur after resecting the diseased or damaged head of femur. The shell is mechanically secured during operation and biologically fastened by in-growth bone surround the femoral neck during recovery period. It eliminates most of the complications from the installation of the existing stem-type hip prostheses.
Another object of present invention is to provide an ideal solution in both the biomechanical and physiological function to the replaced femur in the arthroplasty, in the respect of stress and strength distribution on both the implanted prosthesis and the structures of retention bone. The loading force would be well distributed on the shell and directly conducted into the cortex bone of the femoral shaft through the prosthetic hollow shell.
Still another object of the present invention is to provide an ideal manner to install femoral prosthesis into human body and to eliminate the damage and change on the natural structure of human body in term of its anatomic structure and its blood flow in the proximal area of femur. The level of the resection is just bellowing the level of the femoral head osteotomy. It is an ideal solution for younger (18 years old or up) patient, who has a damaged femoral head. Because of non-stem design of the femoral prosthesis, there are no any damages on intra-medullar canal and bone marrow of proximal femur. The new device would not obstruct the bone growing in young patient, in respect of the bone growth in both the medullar and shaft of femur.
Still another object of the present invention is provide a femoral prosthesis that allows a patient to be substantially completely mobile after the operation and needs a shorter recovery period, because of a simple operative technique and a shorter period of the operation time. Furthermore, such an implanted femoral prosthesis, if necessary, may be easily removed in a number of years following the initial surgery for another surgery.